LPAAT, also referred to as 1-acyl sn-glycerol-3-phosphate acyltransferase (EC 2.3.1.51), is known to catalyze the acylation of lysophosphatidic acid (LPA) to phosphatidic acid (PA) by acylating the sn-2 position of LPA with a fatty acid acyl-chain moiety. LPA and PA, while originally identified as intermediates in lipid biosynthesis (Kent, Anal. Rev. Biochem. 64:315-343, 1995), have more recently been identified as phospholipid signaling molecules that affect a wide range of biological responses (McPhail et al., Proc. Natl. Acad. Sci. USA 92:7931-7935, 1995; Williger et al., J. Biol. Chem. 270:29656-29659, 1995; Moolenaar, Curr. Opin. Cell Biol. 7:203-210, 1995).
Cellular activation in monocytic and lymphoid cells is associated with rapid upregulation of synthesis of phospholipids (PL) that includes PA, diacylglycerol (DAG) and glycan phosphatidylinositol (PI). PAs are a molecularly diverse group of phospholipid second messengers coupled to cellular activation and mitogenesis (Singer et al., Exp. Opin. Invest. Drugs 3:631-643, 1994). PA can be generated through hydrolysis of phosphatidylcholine (PC) (Exton, Biochim. Biophys. Acta 1212:26-42, 1994) or glycan PI (Eardley et al., Science 251:78-81, 1991; Merida et al., DNA Cell Biol. 12:473-479, 1993), through phosphorylation of DAG by DAG kinase (Kanoh et al., Trends Biochem. Sci. 15:47-50, 1990) or through acylation of LPA at the SN2 position (Bursten et al., Am. J. Physiol. 266:C1093-C1104, 1994).
Compounds that block PA generation and hence diminish lipid biosynthesis and the signal involved in cell activation are therefore of therapeutic interest in, for example, the areas of inflammation and oncology as well as obesity treatment. Therefore, compounds that block LPAAT activity have a similar therapeutic value.
The genes coding for LPAAT have been isolated in bacteria (Coleman, Mol. Gen. Genet. 232:295-303, 1992), in yeast (Nagiec et al., J. Biol. Chem. 268:22156-22163, 1993) and in plants (Brown et al., Plant Mol. Biol. 26:211-223, 1994; and Hanke et al., Eur J. Biochem. 232:806-810, 1995; Knutzon, et al., Plant Physiol. 109: 999-1006, 1995). Moreover, two human isoforms of LPAAT have been reported (West, et al., DNA Cell Biol. 6: 691-701, 1997). These isoforms are denominated LPAAT.alpha. and LPAAT.beta. (West, et al., DNA Cell Biol. 6: 691-701, 1997) and are described herein. There remains, however, a need for the isolation of additional mammalian LPAATs, which can be used, for example, to screen for compounds that inhibit LPAAT activity.